I sand blasted mine and then painted it black. My intent was to try to make it blend in a bit. In my opinion that radiator is the least appealing part of the bike and the less attention drawn to it the better.
But, I guess if would make a great canvas for a mural, or a flag. Plenty of surface area to work with.
Mine's matte black. I think the less you can see it the better, although I reckon a dark jack - a bit like this but without the pistons - would look pretty good iof you were going that way.